An error-tracking PWM method is well known as a method for controlling AC output voltage of a voltage source DC-AC power converter used in an Uninterruptible Power System (UPS) and the like (for example, see Masaaki Ohshima, et al., “A Novel Single-Phase UPS Inverter by Error-Tracking Mode PWM Scheme”, Journal of IEEJ D, vol. 120, Jan., 2000, pages 104 to 111, and Masaaki Ohshima, et al., “A Novel Three-Phase UPS Inverter Driven by Error-Tracking-Mode PWM Scheme”, Journal of IEEJ D, vol. 125, No. 2 (2005), pages 164 to 173).
Frequently the voltage source DC-AC power converter includes a transformer in order to insulate a converter and a load or to match output of the converter to system voltage at the load. On the other hand, when a DC component is included in input voltage of a load-side transformer due to a temperature drift or a deviation of an offset of a converter control system, an iron core of the transformer is unidirectionally magnetized by the DC component to generate biased magnetization in which a magnetic flux distribution is biased. This causes problems such as an extreme decrease in excitation inductance of the transformer, a passage of an excessive excitation current, a distortion of AC output voltage, local heating of the transformer, and an increase in electromagnetic noise. Therefore, the AC power supplied from the voltage source DC-AC power converter becomes unstable. For example, a technique of correcting a PWM command using DC component suppressing means for integrating AC output voltage as illustrated in FIGS. 15 and 16 is well known as the method for solving the problems (for example, see Japanese Patent Publication Laid-Open No. 2006-254636). FIG. 15 illustrates a single-phase power converter 211 that produces single-phase AC power from a DC power supply to supply the single-phase AC power to a load connected to an output terminal 222. The single-phase power converter 211 includes a single-phase voltage source DC-AC power converting circuit 240, an inductor Lsp, a voltage detecting circuit 250, and a target current producing means 260. The single-phase voltage source DC-AC power converting circuit 240 converts DC power from a DC terminal 221 to the single-phase AC power. The inductor Lsp is connected to an AC side of the single-phase voltage source DC-AC power converting circuit 240. The voltage detecting circuit 250 detects an output terminal-to-output terminal voltage applied between the output terminals 222. The target current producing means 260 produces target current as a target value of output current at the AC terminal 222. Pulse width modulation control of the single-phase voltage source DC-AC power converting circuit 240 of the converter is performed based on an error between the inverter current and the target current. The target current producing means 260 includes an integrator 271 that amplifies an integration value of the voltage between the output terminals 222, detected by the voltage detecting circuit 250, and a DC component suppressing means 282 that corrects the target current to suppress the DC component included in the single-phase AC power. FIG. 16 illustrates a three-phase power converter 312 that produces three-phase AC power from the DC power supply to supply the three-phase AC power to a load connected to an output terminal 322. The three-phase power converter 312 includes a three-phase voltage source DC-AC power converting circuit 340, an inductor Lsp, a voltage detecting circuit 350, and a target current producing means 360. The three-phase voltage source DC-AC power converting circuit 340 converts DC power from a DC terminal 321 to the three-phase AC power. The inductor Lsp is connected to an AC side of the three-phase voltage source DC-AC power converting circuit 340. The voltage detecting circuit 350 detects an output terminal-to-output terminal voltage applied between the output terminals 322. The target current producing means 360 produces target current as a target value of output current. The pulse width modulation control of the converter is performed based on an error between the inverter current and the target current. The target current producing means 360 includes an integrator 371 that amplifies an integration value of output terminal-to-output terminal voltage detected by the voltage detecting circuit 350 and a correction amount computing means 382 that corrects the target current to suppress the DC component included in the three-phase AC power.